This invention relates generally to the field of fuseable torsion shafts as specifically applied to an aircraft wing flap drive actuation system. The invention details a torque transmitting member that exhibits high strength elastic characteristics during normal operation and then lower strength plastic characteristics after overload. The torque transmitting member inhibits the transmission of excessive torque by partially yielding in torsion at a specific threshold load and afterward maintaining a somewhat lower torque transmission capability for a limited deflection.
In aircraft flight control, actuators are used on the leading edge flaps, the trailing edge flaps, ailerons, flaperons, and rudders in order to connect the flight control surfaces to the frame and allow adjustment of the surfaces to desired positions. The leading edge of an aircraft wing is referred to as a secondary flight control surface since the loss of its use restricts aircraft performance, but does not cripple the aircraft. Generally, mechanical actuators, either linear or rotary, are used on the leading edge of an aircraft. If an actuator jams, it is permissible that the flap be allowed to lock in the jammed position. However, a jammed actuator can cause damage to the aircraft wing if one continues to drive the jammed or remaining actuators. To prevent a jammed actuator from damaging the aircraft, the aircraft may be equipped with a sensing device which, upon sensing asymmetry between the right and left wing flaps, shuts down power to the actuators. This sensing device may also be connected to brakes built into the outboard actuators on each wing, which engage to lock the position of both wing flaps.
There are two problems associated with this type of wing flap actuation system. First, a certain amount of asymmetry between the right and left side wing flaps is allowable and at times unavoidable during certain maneuvers or when flying thru turbulent air. However, it is also desirable to shut down the actuation system as quickly as possible whenever an actuator jams to prevent damage to the aircraft. Thus, the sensing device must be calibrated to balance these conflicting requirements, and prevent unnecessarily shuting down the actuation system. This is accomplished by calibrating the sensing device to shut down the actuation system whenever the right and left side wing flaps are more than a few degrees out of symmetry. Occasionally, however, this small asymmetry limit causes shut down of the actuation system during normal operations, i.e. when there is not a jammed actuator. Similarly, an electronic malfunction can cause inadvertent shutdown. This is referred to as "nuisance braking", and results in loss of the use of the secondary control surfaces.
Secondly, due to the very high rotational speeds at which the actuators are driven, coupled with the delay involved with shutting down the power drive unit, the shafts connecting the power drive unit to the actuators may be driven through as many as approximately ten full revolutions prior to system shutdown. On the wing including the failed actuator, this results in a large torque buildup between the power drive unit and the failed actuator, a portion of this torque potentially acting to twist the wing flap inboard of the failed actuator. After system shutdown, the opposite wing actuation system distributes the torque between the power drive unit and the outboard actuator which includes the brake. This torque buildup may severely damage the remaining actuators, mounting brackets, flaps, wing frame, or drive shafts.
To prevent major damage due to nuisance braking, the drive shaft immediately inside of the outboard actuator may be designed to shear at a specified torque. Thus, the outboard actuator and the flap will be locked by the actuator brake, while the opposite wing flap is skewed up to approximately two degrees by the free actuators. A disadvantage associated with this system is that a nuisance braking event will cause the actuator brakes on both sides to lock, possibly shearing the outboard drive shafts, and locking the position of the flaps, potentially in an undesirable position. Additionally, no protection is provided to the actuator system side which includes the failed or jammed actuator.